Btrfs: take i_mutex before generic_write_checks

btrfs_file_write was incorrectly calling generic_write_checks without
taking i_mutex.  This lead to problems with racing around i_size when
doing O_APPEND writes.

The fix here is to move i_mutex higher.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 17 insertions, 8 deletions

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 1be96ba6f6bb..f155179877a6 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -920,26 +920,35 @@ static ssize_t btrfs_file_write(struct file *file, const char __user *buf,
         start_pos = pos;
 
         vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
+
+        /* do the reserve before the mutex lock in case we have to do some
+         * flushing.  We wouldn't deadlock, but this is more polite.
+         */
+        err = btrfs_reserve_metadata_for_delalloc(root, inode, 1);
+        if (err)
+                goto out_nolock;
+
+        mutex_lock(&inode->i_mutex);
+
         current->backing_dev_info = inode->i_mapping->backing_dev_info;
         err = generic_write_checks(file, &pos, &count, S_ISBLK(inode->i_mode));
         if (err)
-                goto out_nolock;
+                goto out;
+
         if (count == 0)
-                goto out_nolock;
+                goto out;
 
         err = file_remove_suid(file);
         if (err)
-                goto out_nolock;
-
-        err = btrfs_reserve_metadata_for_delalloc(root, inode, 1);
-        if (err)
-                goto out_nolock;
+                goto out;
 
         file_update_time(file);
 
         pages = kmalloc(nrptrs * sizeof(struct page *), GFP_KERNEL);
 
-        mutex_lock(&inode->i_mutex);
+        /* generic_write_checks can change our pos */
+        start_pos = pos;
+
         BTRFS_I(inode)->sequence++;
         first_index = pos >> PAGE_CACHE_SHIFT;
         last_index = (pos + count) >> PAGE_CACHE_SHIFT;